Chris, thank you for your reply and support, especially considering the fact that this mailing list is not intended for this purposes, I really appreciate this.
The OS X installer does require a target disk be GPT. But ultimately it's up to your firmware to be OK with GPT, I suppose if the computer is BIOS based and has issues with GPT, then you wouldn't even get to GRUB, in which case you could convert it to MBR, use gdisk to zap the GPT primary and secondary header+table, and not clear the MBR, and then reinstall grub.
On Apple hardware, Apple's boot loader for OS X is an EFI boot loader. So if your firmware isn't UEFI based, I don't suppose you can use Apple's bootloader, you'd maybe need to use GRUB's xnu bootloader.